The present invention relates to a process for processing an information transmission signal by spread spectrum and a corresponding receiver.
The direct sequence spread spectrum modulation technique has been used for many years, particularly in radio communications with satellites and in the military sector.
In a digital data emitter using a conventional modulation technique, the data to be emitted modulate a radio-frequency carrier. The modulation used can be a phase, frequency, amplitude or mixed modulation. In order to simplify the description, reference will only be made to phase modulations, which are now the most frequently used.
The digital data to be transmitted consist of binary elements or bits, which have a period Tb, i.e. a new bit must be transmitted every Tb. With said bits it is possible to form bit groups, also known as symbols, whose period is Ts and is a multiple of Tb. These symbols will modulate the radio-frequency carrier, e.g. in phase.
This technique can be illustrated by two phase modulation examples:
a) The modulation known as binary phase shift keying or BPSK, which consists of allocating a phase state, e.g. 0, to the 0 bits, and a phase state xcfx80 to the 1 bits. In this case the symbol is the actual bit (Ts=Tb) and the radio-frequency carrier phase state is imposed on every bit.
b) Modulation known as quaternary phase shift keying or QPSK, which consists of using symbols formed by two successive bits, so that said symbols can assume four states (00, 01, 10, 11). A state of the phase of the carrier is allocated to each of these states, in this case Ts=2Tb and the radio-frequency carrier phase state is imposed on every other bit.
On the reception side, it is necessary to demodulate the signal received. A distinction can be made between two major demodulation families, namely coherent demodulation and non-coherent demodulation. The coherent demodulation technique consists of implementing, in the receiver, a subassembly, whose function is to estimate the mean phase of the carrier, so as to reconstitute a phase reference, which is then mixed with the signal received in order to demodulate the data.
The non-coherent demodulation technique is based on the observation, according to which it is sufficient for the phase reference of the symbol to be compared with the phase of the preceding symbol. In this case, instead of estimating the phase of the symbols, the receiver estimates the phase difference between two successive symbols. This is a differential phase shift keying or DPSK or a differential quadrature phase shift keying or DQPSK.
The attached FIGS. 1 to 3 diagrammatically show the structure and operation of a spread spectrum emitter and receiver operating in DPSK. This corresponds to FR-A-2 712 129.
FIG. 1 shows the block diagram of an emitter. Said emitter has an input Ee, which receives the data bk to be emitted and comprises a differential coder 10, constituted by a logic circuit 12 and a delay circuit 14. The emitter also comprises a pseudorandom sequence generator 30, a multiplier 32, a local oscillator 16 and a modulator 18 connected to an output Se, which supplies the DPSK signal.
The logic circuit 12 receives the binary data bk and delivers the binary data dk. The logic circuit 12 also receives the data delayed by one order or rank, i.e. dkxe2x88x921. The logic operation performed in the circuit 12 is the exclusive-OR on the data bk and on the delayed compliment of dk (i.e. on {overscore (dkxe2x88x921+L )}):
dk=bk{circle around (+)}{overscore (dkxe2x88x921+L )}
The pseudorandom sequence used on emission for modulating the data must have an autocorrelation function with a marked peak (of value N) for a zero delay and the smallest possible secondary lobes. This can be obtained by using maximum length sequences, also called m-sequences, or so-called GOLD or KASAMI sequences in exemplified manner. This pseudorandom sequence designated {Cl}, has a bit rate N times higher than the rate of the binary data to be transmitted. The duration Tc of a bit of said pseudorandom sequence and which is also known as a chip is consequently equal to Tb/N.
The chip rate of the pseudorandom sequence can be several million, or several tens of millions per second.
The attached FIG. 2 is the block diagram of a corresponding receiver of the differential demodulator type. This receiver has an input Er and comprises a matched filter 20, whose pulse response is the time reverse of the pseudorandom sequence used in the emitter, a delay circuit 22 with a duration Tb, a multiplier 24, an integrator 26 on a period Tb and a logic decision circuit 28. The receiver has an output Sr, which restores the data.
If x(t) is used for designating the signal applied to the input Er, the multiplier 24 receives the filtered signal xF(t) and the delayed-filtered signal xF(txe2x88x92Tb). The product is integrated on a period equal to or smaller than Tb in the integrator 26, which supplies a signal, whose polarity makes it possible to determine the value of the transmitted bit.
The input filter 20 used in the receiver has a base band equivalent pulse response H(t) and said response must be the time-reverse, conjugate complex of the pseudorandom sequence c(t) used on emission:
H(t)=c*(Tbxe2x88x92t)
The signal supplied by such a filter is consequently:
xF(t)=x(t)*HF(t)
where the symbol * designates the convolution operation, i.e.
xF(t)=∫0Tbx(s).c*(sxe2x88x92t)ds.
Thus, the matched filter 20 performs the correlation between the signal applied at its input and the pseudorandom spread sequence.
In a gaussian additive noise channel, the signal x(Ft) will consequently be in the form of a pulse signal, the pulse repetition frequency being 1/Tb. The envelope of this signal is the autocorrelation function of the signal c(t). The information is carried by the phase difference between two successive correlation peaks. Thus, the multiplier output is formed by a succession of positive or negative peaks, as a function of the value of the transmitted bit.
In the case of a radio transmission in the presence of multiple paths, the output of the matched filter is formed by a succession of correlation peaks, each peak corresponding to a propagation path.
The different signals of the reception chain are represented in FIG. 3.
Line (a) represents the filtered signal xF(t), line (b) the correlation signal xFt)*xF(txe2x88x92Tb) and line (c) the signal at the integrator output.
The direct sequence spread spectrum modulation technique has been extensively described in the specialist literature and reference can e.g. be made to the following works:
xe2x80x9cCDMA Principles of Spread Spectrum Communicationxe2x80x9d, by Andrew J. VITERBI, Addison-Wesley Wireless Communications Series,
xe2x80x9cSpread Spectrum Communicationsxe2x80x9d, by Marvin K. SIMON et al., vol. I, 1983, Computer Science Press,
xe2x80x9cSpread Spectrum Systemsxe2x80x9d, by R. C. DIXON, John WILEY and Sons.
This technique is also described in certain articles:
xe2x80x9cDirect-sequence Spread Spectrum with DPSK Modulation and Diversity for Indoor Wireless Communicationsxe2x80x9d, published by Mohsen KAVEHRAD and Bhaskar RAMAMURTHI in the journal xe2x80x9cIEEE Transactions on Communicationsxe2x80x9d, vol. COM 35, No. 2, February 1987,
xe2x80x9cPractical Surface Acoustic Wave Devicesxe2x80x9d, by Melvin G. HOLLAND, in the journal Proceedings of the IEEE, vol. 62, No. 5, May 1974, pp 582-611.
The direct sequence spread spectrum technique has numerous advantages, such as:
Discretion: this discretion is linked with the spread of the transmitted information over a wide frequency band, leading to a low spectral density of the emitted power.
Multiple access: several direct sequence spread spectrum links can share the same frequency band using orthogonal spread pseudorandom sequences (sequences having an intercorrelation function having very low residual noise for all shifts), said technique being known as code distribution multiple access or CDMA.
A good cohabitation with conventional narrow band communications: the same frequency band being shared by systems using a narrow band modulation and those using a broad band modulation. There is only a slight increase in ambient radio noise to narrow band communications and this decreases with the increase in the sequence length. Spread spectrum modulation communications bring about a rejection of narrow band modulations due to the correlation operation performed on reception.
The interception difficulty: a direct sequence spread spectrum transmission is difficult to intercept as a result of the low spectral density and the fact that the receiver must know the spread sequence in order to be able to demodulate the data.
An excellent behaviour in a multi-path environment, where the propagation of the radio wave takes place in accordance with multiple paths using reflection, diffraction and scattering phenomena. Moreover, not infrequently there is no longer a time-stable, direct path between the emitter and the receiver. This multiple path propagation induces parasitic effects, which tend to deteriorate the transmission quality.
The technique described hereinbefore suffers from a disadvantage linked with the need to perform an estimate of the channel, i.e. a determination of the number of paths followed by the radio wave, with the corresponding delays and amplitudes, this applying to each user. This estimate is often difficult, particularly when the channel has multiple paths, there are several users and a certain radio noise level interferes with the signals.
Moreover, in an integration receiver of the type described hereinbefore, the energy carried by the multiple paths is summated by means of an accumulator, which functions on a part of the symbol. In order that such receivers function in a satisfactory manner, it is necessary for the pulse response of the channel (part where the peaks appear) is shorter than the duration of a symbol (generally twice shorter). If this rule is not respected, it becomes difficult to separate the symbols, and when the pulse response is greater than the symbol, an intersymbol distortion phenomenon prevents a correct data demodulation. The accumulation principle no longer gives a good performance level.
The object of the present invention is to obviate these disadvantages.
To this end, the invention proposes a process no longer operating in accordance with the signal integration principle, but which instead processes successively the correlation peaks in decreasing amplitude order. For this purpose, the process according to the invention consists of determining the highest amplitude correlation peak, subtracting the information corresponding to this peak from the signal to be processed and again processing the signal in the same way and so on until the peaks are exhausted. On the basis of the successive informations obtained by the different passes of this iteration, the transmitted information is restored.
In other words, instead of considering the different correlation peaks in a global manner by integrating them, they are successively, individually analyzed.
More specifically, the present invention relates to a process for the processing of a signal corresponding to a direct sequence spread spectrum information transmission through a multiple path channel, said process being characterized in that it comprises the following operations:
a) a correlation is performed between the signal to be processed and the direct sequence used, which supplies several correlation peaks of different amplitudes,
b) the highest amplitude correlation peak is determined and the corresponding information restored,
c) the spectrum of the information corresponding to the highest amplitude peak is respread using the same direct sequence,
d) from the signal to be processed is subtracted the part corresponding to said respread information and a new signal is obtained,
e) operations a, b, c and d are reiterated for said new signal and so on until the peaks are exhausted,
f) on the basis of the different informations successively obtained for the different correlation peaks, the transmitted information is restored.
The present invention also relates to a direct sequence spread spectrum receiver for the successive processing of correlation peaks corresponding to different transmission paths, for the performance of the process defined hereinbefore. This receiver comprises at least two circuits in cascade, each having:
an adder able to receive an input signal,
means for performing a correlation between a pseudorandom sequence and the input signal, said means supplying several correlation peaks,
means for determining the highest amplitude peak and for restoring the corresponding information,
means for respreading the information obtained corresponding to the highest amplitude peak and for obtaining an output signal of appropriate polarity,
means for delaying the input signal by a time equal to the output signal formation time, said output signal and said delayed signal of one of the circuits constituting the input signal of the following circuit.